1. Field of the Invention
The present invention relates to the optical alignment of solar concentrators, more particularly, to a method and apparatus for the alignment of solar concentrators using two lasers.
2. Related Art
A variety of techniques exist for providing a solar concentrator alignment. These techniques include on-sun alignment, distant observer alignment, distant light source alignment, conventional laser alignment, and near light source alignment.
In on-sun alignment, individual mirrors are aligned while the concentrator tracks the sun. This technique is limited in usefulness in that it must be performed during sunny days and any error in tracking can translate into significant alignment errors.
With the distant observer approach, a xe2x80x9cbulls-eyexe2x80x9d type target, located near the focus of the concentrator, is observed in a reflection from the concentrator at a distance. Mirrors are individually adjusted to maximize the xe2x80x9cbulls-eyexe2x80x9d color seen in the concentrator by the observer.
In the distant light source technique, the concentrator is illuminated by a lamp located at a known distance of approximately 500 to 2000 feet along the optical axis of the concentrator. The location and shape of the reflected images on a target, of specified location near the focus of the concentrator, is then adjusted to correspond to the calculated locations drawn on the target. This technique is limited by the fact that it can only be performed at night.
In conventional laser alignment, a laser is located near the xe2x80x9c2fxe2x80x9d location of the concentrator, i.e., one focal length behind the concentrator focal point. As in the distant light source technique, the mirrors are adjusted to align the reflected laser beam on calculated, predetermined locations on a target board, as located near the xe2x80x9c2fxe2x80x9d location. Laser alignment can be accomplished day or night.
The near light source technique is similar to conventional laser alignment with respect to target and light source location. It differs in that a lamp is used instead of a laser, and the entire concentrator is illuminated. However, because of the overlap of reflected images on the target, this approach is limited to concentrators that can be defocused.
These abovementioned techniques are described in more detail in Richard B. Diver, xe2x80x9cMirror Alignment Techniques for Point-Focus Solar Concentrators,xe2x80x9d SAND92-0668, Sandia National Laboratories, Albuquerque, N.Mex., June, 1992 and Richard B. Diver, xe2x80x9cMirror Alignment and Focus of Point-Focus Concentrators,xe2x80x9d Proceedings of the 1995 ASME/JSME/JSES, International Solar Energy Conference, Maui, Hi., March, 1995.
Of the techniques described above, only the conventional laser technique represents a practical approach for most commercial systems. The laser technique can be performed both day and night and is not hindered by line-of-sight access limitations-typically seen in a large field of dishes where some dishes"" line-of-sight access will be completely blocked by surrounding dishes. However, the conventional laser technique also has some disadvantages. These disadvantages include misalignments due to the effect of level slope errors when only a single alignment sampling takes place. Also, the conventional laser technique requires exact positioning of the alignment target, along with precise knowledge of concentrator facets and aim point coordinates. Furthermore, variations between individual solar concentrators can result in the requirement that unique targets must be made for each one-resulting in added expense.
Based on the limitations of the aforementioned alignment techniques, their remains a need for a solar concentrator alignment technique that (1) is simple to set up and implement, (2) uses a minimal amount of sophisticated hardware, (3) does not require the removal of existing hardware on the concentrator, (4) does not require use of the sun or restrictive weather conditions, (5) does not require line-of-sight to a distant observer or distant light source and (6) permits real-time alignment of concentrator facets while the concentrator remains in operation.
In accordance with the invention, a method and apparatus are provided which use a laser alignment approach but which provides important advantages over conventional laser alignment systems and methods. As discussed in more detail below, the method and apparatus of the invention share the important practical advantages of conventional laser alignment techniquesxe2x80x94including the ability to perform during both the day and nightxe2x80x94while eliminating or greatly reducing problems associated with conventional laser techniques.
In accordance with one aspect of the invention, an apparatus is provided for aligning a solar concentrator having a plurality of facets, the concentrator defining a focal point and an optical axis having a vertex, and the apparatus comprising: a first laser for producing a first laser beam for targeting a selected facet of the plurality of facets of the concentrator; a target board positioned adjacent to the first laser at approximately one focal length behind the focal point of the concentrator such that the target board will be illuminated by the first laser beam from the first laser after reflection of the first laser beam off of the selected facet; and a second laser, located adjacent to the vertex of the optical axis of the concentrator, for producing a second laser beam directed at the target board at a target point thereon such that by adjusting the selected facet to cause the first laser beam to illuminate the target point on the target board produced by the second laser beam, the selected facet can be brought into the desired alignment.
Preferably, the apparatus further comprises a first steerable mirror associated with the first laser for steering the first laser beam and a second steerable mirror associated with the second laser for steering the second laser beam.
Advantageously, the first and second laser beams produced by the first and second lasers are of different colors so as to enable the beams to be distinguished from one another.
In accordance with a further aspect of the invention, a method is provided for aligning a solar concentrator having a plurality of facets, the concentrator defining a focal point and an optical axis having a vertex, and the method comprising: (a) using a first laser to direct a first laser beam onto a selected facet of the plurality of facets of the concentrator such that a target board positioned adjacent to the first laser at approximately one focal length behind the focal point of the concentrator is illuminated by the first laser beam after reflection thereof off of the selected facet; (b) using a second laser, located adjacent to the vertex of the optical axis of the concentrator, to direct a second laser beam onto the target board at a target point thereon; (c) adjusting the selected facet to cause the first laser beam to illuminate the target point on the target board produced by the second laser beam, so as to bring the selected facet into alignment with the target point; and repeating steps (a) to (c) for other selected facets of the concentrator, as necessary, to provide the desired alignment of the concentrator. It should be noted that by using this method, the resulting alignment could correspond with the focal point of the concentrator, or if desired, due to operational considerations, could result in individual facets being aligned in order to tailor the energy flux distribution at the concentrator focal point.
As above, the method preferably comprises using a first steerable mirror for directing the first laser beam and a second steerable mirror for directing the second laser beam, and the first and second laser beams produced by the first and second lasers are preferably of different, distinguishable colors.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.